The invention concerns an automatic clutch in the drive train of a motor vehicle, which has a transmission which is shifted manually between different drive positions or transmission stages. A motorized adjusting unit actuates the clutch and is controlled by a system of sensors which monitors predetermined parameters and releases the clutch if criteria for a change of the transmission stage intended by the driver are detected by the system of sensors. When the transmission stage has been selected, the clutch is controlled in analogous with the torque of the engine with excess contact pressure such that the moment which can be transmitted by the clutch is a predetermined amount above the moment which can be picked up at the engine.
In motor vehicles with customary internal combustion engines, a transmission must be arranged in the drive train to allow the transmission ratio between the speed of the vehicle engine and the speed of the drive wheels to be changed according to the respective travelling speed and loading of the vehicle. In the case of customary manually shifted transmissions, during a change of drive position the power flow between the engine and the drive wheels has to be interrupted by releasing the clutch.
It is known in principle to use automatic clutches for this purpose, i.e. the clutch is automatically released when changing a transmission stage and is subsequently re-engaged. During release of the clutch, spring elements which non-positively clamp the clutch parts on the input and output sides against one another are relieved by the travel of the adjusting unit. During subsequent engagement of the clutch, the spring elements are again clamped with varying intensity to produce an adherent engagement of the clutch.
To minimize the travel of the adjusting unit necessary for releasing the clutch, it is known in principle to keep the clutch engaged with an excess contact pressure changed in an analogous fashion with the torque of the engine, i.e. said spring elements are in each case clamped only with such a level of intensity that the adherent engagement of the clutch allows the transmission of a moment which lies above the respective engine moment only by a limited amount. This is synonymous to saying that the clamping of the spring elements is generally significantly less than the maximum allowed by design.
The limited excess contact pressure in principle offers the advantage that any shocks that there may be in the drive train can lead to a "slipping" of the clutch and consequently be effectively absorbed.
On the other hand, the excess contact pressure must not be too low in order to avoid undesired wearing of the clutch.
The object of the invention is thus to improve the control of the excess contact pressure.
This object is achieved according to the invention by reducing the amount of excess contact pressure at low outside temperatures.
The invention is based on the general idea of controlling the excess contact pressure in a temperature-dependent manner, and of reducing it at low ambient temperatures in order to reduce the energy necessary for releasing the clutch when the adjusting unit has to work against an increased resistance on account of low ambient temperature. The adjusting unit generally operates hydraulically, so that an increased viscosity of the hydraulic medium must be expected at low temperature, i.e. the adjusting unit becomes sluggish.
According to the invention, the energy-increasing effect of the increasing viscosity of the hydraulic medium of the hydraulic clutch adjusting unit as temperatures fall can be compensated, so that the power requirement for the adjusting unit remains limited. Consequently, it can be reliably ensured that the clutch is released quickly when intended shifting by the driver is detected. Quick releasing of the clutch is desired because an interruption of the power flow between the engine and the drive wheels can then take place, and neither the actuating elements for shifting the transmission nor the clutch linings can be excessively loaded, even when the driver changes the transmission stages very quickly, i.e. if the driver "rips through" the gears of the transmission.
Otherwise, with regard to preferred features of the invention, reference is made to the claims and the following explanation of the drawing, on the basis of which particularly preferred embodiments of the invention are described.